This invention relates to methods of fabricating integrated microelectronic circuits; and more particularly to, methods of compensating for proximity effects which occur when a layer of electron sensitive resist is patterned with a scanning electron beam. During the fabrication of the above mentioned microelectronic circuits, it is common practice to cover a layer of some material which is to be patterned (such as an insulating layer of silicon dioxide, or a conductive layer of metal) with a layer of electron sensitive resist. Subsequently, various portions of the resist which correspond to the pattern that is to be formed on the underlying material are bombarded with a scanning electron beam. This changes the solubility of the resist in the bombarded regions. Consequently, either the bombarded portions of the resist or the unbombarded portions may be selectively removed by exposing the resist to a suitable etchant. Those portions of the resist which thereafter remain are then used as a mask for patterning the underlying material.
One limitation however to the above technique is caused by electron scattering. As used herein electron scattering refers to the fact that as the electron beam passes through the resist, some of those electrons collide with the molecules of the resist and are thereby scattered in various directions. Thus, the amount of energy which a particular point in the resist receives is not simply a function of the energy of the overlying electron beam; but it is also a function of the energy and all of the electron beams which are in close proximity to that point. This is a problem because all of the points in the resist portions that are bombarded do not receive the same number of scattered electrons. Consequently, the solubility of the bombarded resist portions varies from one point to another, and this limits the fidelity with which the resist can be patterned.
In the past, one technique for dealing with this problem has been to modify the shape of the portions of the resist that are bombarded with electrons from those shapes which are actually desired. For example, if it is desired to shape the resist into a long narrow line that is only 0.5 micrometers wide, then this might be achieved by bombarding two portions of resist that are spaced apart by 0.70 micrometers, as opposed to 0.50 micrometers. Conversely, if it were desired to form a long narrow gap of 0.5 micrometers width in the resist, then this might be achieved by bombarding a long narrow portion of the resist which is only 0.3 micrometers wide as opposed to 0.5 micrometers wide. In each case, the exact amount by which the bombarded shape must be modified to achieve the desired shape depends upon not only the shape of the particular resist portion but also upon the resist thickness, the beam energy, and the proximity of each resist portion to all of the other portions that are being bombarded.
Another technique which has been used in the prior art to compensate for electron scattering has been to keep the shape of each portion of the resist which is bombarded the same as the shape that is desired, but to vary the dosage of electrons which each particular shape in the pattern receives. That is, one portion of the resist receives a uniform dosage D.sub.1 ; another portion of the resist receives a uniform dosage D.sub.2 ; etc. But this technique is also undesirable because the dosage of electrons for any particular region depends upon the shape of that region, the proximity of that region to other regions which are being bombarded, the resist thickness, and the beam energy. And since typical microelectronic circuit patterns contain many thousands of shapes in millions of combinations with each other, correction by this technique is extremely complicated and can be achieved only through large amounts of computer calculation and experimental optimization.
Therefore, it is a primary object of this invention to provide an improved method of compensating for proximity effects which occur in electron beam lithography.
Still another object of the invention is to provide a method of compensating for proximity effects which is simple, is valid for all pattern shapes and combinations, works for a large variety of process conditions, and does not require any complex computer calculations.